System and method for magnetizing a transformer in an electrical system prior to energizing the electrical system

ABSTRACT

An electrical system ( 2 ) includes a transformer ( 16 ) coupled to an AC source ( 6 ) that provides a main AC voltage, the transformer having a number of sets of primary windings ( 18 ) and secondary windings ( 20 ), and a charging module ( 32 ) structured to generate a magnetizing AC voltage. The charging module is structured to selectively provide the magnetizing AC voltage to: (i) one of the number of sets primary windings, or (ii) one of the number of sets secondary windings. The magnetizing AC voltage is such that responsive to the magnetizing AC voltage being provided to one of the sets of primary windings or one of the sets of secondary windings, one or more of the sets of primary windings will be magnetized in a manner wherein a flux of the one or more of the number of primary windings is in phase with the main AC voltage provided from the AC source.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from and claims the benefit of U.S.patent application Ser. No. 14/570,377, filed Dec. 15, 2014, which isincorporated by reference herein.

BACKGROUND Field

The disclosed concept pertains generally to electrical systems thatemploy AC transformers, such as, without limitation, a variablefrequency drive employing an isolation transformer or an electricaldistribution system employing a distribution transformer, and, moreparticularly, to a system and method for magnetizing the transformerprior to energizing the electrical system from the main AC source.

Background Information

A voltage source inverter is often used to power a motor, such as aninduction or synchronous motor, or a generator, with a suitable mediumvoltage. One example of a voltage source inverter is a variablefrequency drive (VFD), which controls the rotational speed of analternating current (AC) electric motor by controlling the frequency ofthe electrical power supplied to the motor. VFDs are also known asadjustable frequency drives (AFDs), variable speed drives (VSDs), ACdrives, microdrives or inverter drives. Since the voltage is variedalong with the frequency, these are sometimes also called VVVF (variablevoltage variable frequency) drives.

Typically, a VFD first converts an AC input power to a DC intermediatepower. The DC intermediate power is then converted to a quasi-sinusoidalAC power for driving the motor. Thus, the main components of a typicalVFD include a number of input isolation transformers coupled to thesource of AC power, a converter, such as a number of rectifier bridgeassemblies, for converting the AC source power into the DC intermediatepower, a direct current (DC) bus and associated DC bus capacitors forstoring the DC intermediate power, and an inverter for converting thestored DC intermediate power into a variable voltage, variable frequencyAC voltage for driving the motor.

One problem encountered by VFDs is caused by the fact that, when atransformer is first energized, a transient current up to 10 to 15 timeslarger than the rated transformer current can flow for several cycles.This transient current is known as inrush current. The magnitude of theinrush current may cause fuses to open, breakers or contactors to open,and protection relays to “false trip”. For large drives, this problem issignificant in that the power system must be sized to provide thetransient in-rush currents. Eliminating the inrush is of significantadvantage as it increases reliability and/or reduces system cost.

A second problem encountered by Voltage Source Inverters is charging thelarge capacitors during initial energization to prevent damage torectifier, fuses and associated circuitry.

The above-described problem of inrush current is not limited to VFDs.Rather, inrush current is a problem for any electrical system thatutilizes a (large) transformer, such as, without limitation, anelectrical distribution system that employs a distribution transformeror any industrial equipment that employs a drive having an inputtransformer. There also needs to be a method of pre charging thecapacitors.

There is thus a need for a system and method for effectively reducingand/or eliminating inrush current in electrical systems that utilizeinput transformers.

SUMMARY

In one embodiment, an electrical system is provided that includes atransformer structured to be selectively coupled to an AC source thatprovides a main AC voltage, the transformer having a number of sets ofprimary windings and a number of sets of secondary windings, and acharging module structured to generate a magnetizing AC voltage. Thecharging module is structured to selectively provide the magnetizing ACvoltage to: (i) one of the number of sets primary windings, or (ii) oneof the number of sets secondary windings. The magnetizing AC voltage issuch that responsive to the magnetizing AC voltage being provided to oneof the number of sets of primary windings or one of the number of setsof secondary windings, one or more of the number of sets of primarywindings will be magnetized in a manner wherein a flux of the one ormore of the number of primary windings is in phase with the main ACvoltage provided from the AC source.

In one embodiment, a method of energizing an electrical system isprovided, wherein the electrical system includes a transformerstructured to be selectively coupled to an AC source that provides amain AC voltage, the transformer having a number of sets of primarywindings and a number of sets of secondary windings. The method includesgenerating a magnetizing AC voltage when the number of sets of primarywindings is not coupled to the AC source, providing the magnetizing ACvoltage to one of the number of sets primary windings or one of thenumber of sets secondary windings when the number of sets of primarywindings is not coupled to the AC source to magnetize one or more of thenumber of sets of primary windings in a manner wherein a flux of the oneor more of the number of primary windings is in phase with the main ACvoltage, and coupling the number of sets of primary windings to the ACsource such that the main AC voltage is applied to the number of sets ofprimary windings.

In another embodiment, a variable frequency drive system is provided.The variable frequency drive system includes a variable frequency driveincluding a transformer structured to be selectively coupled to an ACsource that provides a main AC voltage, the transformer having a numberof sets of primary windings and a number of sets of secondary windings,a converter coupled to the number of sets of secondary windings, a DClink coupled to an output of the converter, and an inverter coupled tothe DC link. The variable frequency drive system also includes acharging module structured to generate a magnetizing AC voltage, whereinthe charging module is structured to selectively provide the magnetizingAC voltage to: (i) one of the number of sets primary windings, or (ii)one of the number of sets secondary windings, wherein the magnetizing ACvoltage is such that responsive to the magnetizing AC voltage beingprovided to one of the number of sets of primary windings or one of thenumber of sets of secondary windings, one or more of the number of setsof primary windings will be magnetized in a manner wherein a flux of theone or more of the number of primary windings is in phase with the mainAC voltage provided from the AC source.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the disclosed concept can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of an electrical system according to onenon-limiting exemplary embodiment which implements a method for reducingand/or eliminating inrush current according to the disclosed concept;

FIG. 2 is a schematic representation illustrating how the windings ofthe auxiliary transformer may be connected to the windings of the maininput transformer of the system of FIG. 1 according to an exemplaryembodiment of the disclosed concept; and

FIG. 3 is a schematic diagram of an electrical distribution system 42according to an alternative exemplary embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Directional phrases used herein, such as, for example, left, right,front, back, top, bottom and derivatives thereof, relate to theorientation of the elements shown in the drawings and are not limitingupon the claims unless expressly recited therein.

As employed herein, the term “number” shall mean one or an integergreater than one (i.e., a plurality).

As employed herein, the statement that two or more parts are “coupled”together shall mean that the parts are joined together either directlyor joined through one or more intermediate parts.

As used herein, the term “set of windings” shall mean a group of one ormore windings such as a group of one or more primary windings or a groupof one or more secondary windings.

The disclosed concept provides a system and method for reducing and/oreliminating inrush current in electrical system by charging ormagnetizing an input transformer, such as, without limitation, anisolation transformer of a VFD, of the electrical system before theelectrical system is energized by a main AC source (e.g., such as themain electrical grid). In particular, and as described in greater detailherein in the various exemplary embodiments, the disclosed conceptprovides a system and method wherein a transformer is charged ormagnetized in advance of the system being fully energized in such amanner that the flux and voltage of the primary winding or windings ofthe transformer are in phase with the main AC source that is soon to beapplied to the transformer.

FIG. 1 is a schematic diagram of an electrical system 2 according to onenon-limiting exemplary embodiment which implements the method forreducing and/or eliminating inrush current of the disclosed concept. Asseen in FIG. 1, system 2 includes a variable frequency drive 4 that isfed by a main AC source 6, such as the main electrical grid, through anisolation switch 8, main fuses 10, and a main contactor 12. In thenon-limiting exemplary embodiment, main AC source 6 is a 4160V,poly-phase (e.g., three-phase) AC input. Also in the non-limitingexemplary embodiment, variable frequency drive 4 is used to drive apoly-phase motor 14.

Variable frequency drive 4 includes a 3-phase, phase shifting maintransformer 16. In the non-limiting, exemplary embodiment, maintransformer 16 is a wye-delta transformer having a set of wye-connectedprimary windings 18 and a number of sets of delta-connected secondarywindings 20. In the exemplary embodiment, main transformer 16 is a24-pulse transformer and includes four sets of delta-connected secondarywindings 20, labeled 20A, 20B, 20C, and 20D. In the non-limiting,exemplary embodiment, each set of delta-connected secondary windings 20comprises a set of extended delta windings, and the voltage atdelta-connected secondary winding 20A is phase shifted +22.5°, thevoltage at delta-connected secondary windings 20B is phase shifted−7.5°, the voltage at delta-connected secondary windings 20C is phaseshifted +7.5°, and the voltage at delta-connected secondary windings 20Dis phase shifted −22.5°. As seen in FIG. 1, a converter 22 is coupled todelta-connected secondary windings 20A-20D and receives the 3-phase ACoutput thereof. Converter 22 has four AC to DC rectifier bridges 24,labeled 24A, 24B, 24C and 24D, arranged in series connection creatingtwo twelve pulse rectifiers which result in 24-pulse harmonic mitigationon the primary of main transformer 16. Converter 22 thus converts the3-phase AC output present on delta-connected secondary windings 20A-20Dto DC power.

The output of converter 22 is coupled to a DC link 26 (sometimes alsoreferred to as a DC bus) having capacitors 28A and 28B. The output of DClink 26 is coupled to the input of an inverter 30. In the exemplaryembodiment, inverter 30 is a 3-level inverter such as a 3-level NPCinverter, although it will be understood that other suitable invertertopologies may also be used. As is known in the art, inverter 30converts the DC power on DC link 26 to 3-phase quasi-sinusoidal AC power(see phases U, V, W in FIG. 1) which is provided to poly-phase motor 14.

Electrical system 2 further includes a 3-phase, phase shifting auxiliarytransformer 32 which, as described herein, is used to magnetize maintransformer 16 of variable frequency drive 4 before variable frequencydrive 4 is energized by main AC source 6 in order to reduce and/oreliminate the inrush current into variable frequency drive 4. The phaseshifting of auxiliary transformer 32 is chosen so as to match the phaseshifting of main transformer 16. Auxiliary transformer 32 iselectrically connected between main fuses 10 and main contactor 12through a fuse 34. Thus, auxiliary transformer 32 is structured toreceive, on the primary thereof, the voltage from main AC source 6. Inthe non-limiting, exemplary embodiment, auxiliary transformer 32 is adelta-wye transformer having a set of delta-connected primary windings36 and a set of wye-connected secondary windings 38. In the exemplaryembodiment, auxiliary transformer 32 is a step down transformer thatconverts the voltage from main source 6 to a lower voltage. In thenon-limiting exemplary embodiment, auxiliary transformer is structuredto output approximately 300V AC on the set of wye-connected windings 38when a 4160V AC voltage is applied to delta-connected primary windings36. It will be understood, however, that this is meant to be exemplaryonly and that other transformer ratios may also be employed within thescope of the disclosed concept.

As seen in FIG. 1, wye-connected secondary windings 38 are coupled to afirst side of a 3-phase auxiliary contactor 40. In the non-limiting,exemplary embodiment, auxiliary contactor 40 is a low voltage contactor.The second side of auxiliary contactor 40 is coupled to one of the setsof delta-connected secondary windings 20 of main transformer 16. In theexemplary embodiment, the second side of auxiliary contactor 40 iscoupled to the set of delta-connected secondary windings 20D, althoughit will be understood that this is exemplary only and that theconnection just described may be made to any of the other sets ofdelta-connected secondary windings 20, or even to the set ofwye-connected primary windings 18.

FIG. 2 is a schematic representation illustrating how the connection ofthe set of wye-connected secondary windings 38 is connected to the setof delta-connected secondary windings 20D through auxiliary contactor 40according to an exemplary embodiment. The set of wye-connected secondarywindings 38 includes windings 38 a, 38 b, and 38 c, and thedelta-connected secondary windings 20D includes extended windings 20Da,20Db and 20Dc. As seen in FIG. 2, winding 38 a is connected at thejunction of winding 20Da and 20Db, winding 38 b is connected at thejunction of winding 20Dc and 20Db, and winding 38C is connected to thejunction of winding 20Dc and 20Da.

Again, it will be appreciated that the particular configurationsdescribed herein are exemplary only, and that other connectionconfigurations are possible within the scope of the disclosed concept.For example, and without limitation, main transformer 16 may be atransformer other than a wye-delta transformer and auxiliary transformer32 may be a transformer other than a delta-wye transformer.

In operation, when variable frequency drive 4 is to be “turned on”, maincontactor 12 is moved to an open position and auxiliary contactor 40 ismoved to a closed position. Isolation switch 8 may then be closed, whichcauses the voltage of main AC source 6 to be applied to the set ofdelta-connected primary windings 36 of auxiliary transformer 32. Thiswill result in a voltage being induced in the set of wye-connectedsecondary windings 38 of auxiliary transformer 32. That voltage will beapplied to the set of delta-connected secondary windings 20D of maintransformer 16 through auxiliary contactor 40 in order to magnetize maintransformer 16. Because of the relatively high impedance of auxiliarytransformer 32, main transformer 16 will be magnetized softly at lessthan the rated current. Once main transformer 16 is sufficientlymagnetized, main contactor 12 is closed such that the voltage of main ACsource 6 will be applied to the already magnetized set of wye-connectedprimary windings 18 of main contactor 16. After main contactor 12 isclosed, then auxiliary contactor 40 is opened. When main contactor 12 isclosed, the phase of the set of wye-connected primary windings 18 willmatch the phase of the voltage of main AC source 6 being applied.Because the wye-connected primary windings 18 have already beenmagnetized as just described, the inrush current into variable frequencydrive 4 will be reduced and/or eliminated. When auxiliary contactor 40is closed, DC link 26 (the DC bus) is charged.

The determination as to when the main transformer 16 is sufficientlymagnetized such that full energizing of variable frequency drive 4 maybegin may be made in any of a number of ways, including monitoring thevoltage of the DC link 26 and determining that sufficient magnetizationhas occurred when that voltage reaches a certain threshold level,monitoring the voltage of the set of wye-connected primary windings 18and determining that sufficient magnetization has occurred when thatvoltage reaches a certain threshold level, or measuring the currentflowing into auxiliary transformer 32 and determining that sufficientmagnetization has occurred when that current settles, meaning that it isno longer changing to a significant degree.

Thus, the disclosed concept provides a mechanism and methodology bywhich a transformer, such as main transformer 16, may be magnetized inadvance of being fully energized in a manner that eliminates and/orreduces the inrush current into the transformer. A secondary benefit ofthe mechanism and methodology of the disclosed concept is that DC link26 will also be charged, thus eliminating the need for a pre-chargecircuit. Furthermore, by adding additional windings to auxiliarytransformer 32, it may be used for other purposes, such as providingpower for a cooling fan for variable frequency drive 4. Still otherpotential benefits include reduced arc flash incident energy levelsbecause protection relays can be set with lower instantaneous currenttrip settings. This feature provides quicker fault clearing time andlower arc flash ratings for the equipment and personnel protectiveequipment.

FIG. 3 is a schematic diagram of an electrical distribution system 42according to an alternative exemplary embodiment. Electricaldistribution system 42 is similar to electrical system 2 describedelsewhere herein, and like components are labeled with like referencenumerals. However, while electrical system 2 employs the disclosedconcept in connection with magnetizing an isolation transformer feedinga variable frequency drive, electrical distribution system 42 employsthe disclosed concept in connection with magnetizing a main distributiontransformer of electrical distribution system 42 for feeding a number ofloads. In particular, as seen in FIG. 3, electrical distribution system42 includes main AC source 6 as described herein, which, in theexemplary embodiment, is a 4160V, 60 Hz utility source, main transformer16 as described herein, auxiliary transformer 32 as described herein,and auxiliary contactor 40 as described herein. Electrical distributionsystem 42 further includes a line breaker 44, which may be a contactor,a fused switch or a circuit breaker, a secondary breaker 46, and a load48. Line breaker 44 is provided between the main AC source 6 and maintransformer 16, and secondary breaker 46 is provided between maintransformer 16 and load 48. In operation, in order to provide power toload 48, line breaker 44 and secondary breaker 46 are in an opencondition. Auxiliary transformer 32 is then powered from main AC source6. Auxiliary contactor 40 then connects the secondary of auxiliarytransformer 32 to the secondary of main transformer 16. Line breaker 44then closes with no inrush current. Next, auxiliary contactor 40 isopened and secondary breaker 46 is closed.

While specific embodiments of the disclosed concept have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the disclosedconcept which is to be given the full breadth of the claims appended andany and all equivalents thereof.

What is claimed is:
 1. An electrical system, comprising: a transformerstructured to be selectively coupled to an AC source that provides amain AC voltage having a main AC voltage level, the transformer having anumber of windings; and a charging module structured to be selectivelycoupled to the AC source that provides the main AC voltage to receivethe main AC voltage having the main AC voltage level and to generate amagnetizing AC voltage in response to receiving the main AC voltagehaving the main AC voltage level from the AC source, wherein thecharging module is structured to selectively provide the magnetizing ACvoltage to the transformer, wherein the magnetizing AC voltage is suchthat responsive to the magnetizing AC voltage being provided to thetransformer one or more of the windings will be magnetized in a mannerwherein a voltage of the one or more of the windings is in phase withthe main AC voltage provided from the AC source to eliminate or reducein-rush current into the transformer.
 2. The electrical system accordingto claim 1, wherein the number of windings comprises a number of sets offirst windings and a number of sets of second windings, wherein thecharging module is structured to selectively provide the magnetizing ACvoltage to: (i) one of the number of sets first windings, or (ii) one ofthe number of sets second windings.
 3. The electrical system accordingto claim 1, wherein the charging module comprises an auxiliarytransformer (32) structured to be selectively coupled to thetransformer.
 4. The electrical system according to claim 3, wherein theauxiliary transformer is structured to be selectively coupled to thetransformer through an auxiliary contactor.
 5. The electrical systemaccording to claim 4, wherein the transformer is a 3-phase phaseshifting wye-delta transformer and the auxiliary transformer is a3-phase shifting delta-wye transformer.
 6. The electrical systemaccording to claim 1, wherein the transformer is structured to beselectively coupled to the AC source through a main contactor.
 7. Theelectrical system according to claim 1, wherein the electrical system isa variable frequency drive system, and wherein the transformer is anisolation transformer of a variable frequency drive.
 8. The electricalsystem according to claim 1, wherein the number of sets of firstwindings is a number of sets of primary windings and the number of setsof second windings is a number of sets of secondary windings, whereinthe charging module is structured to selectively provide the magnetizingAC voltage to: (i) one of the number of sets primary windings, or (ii)one of the number of sets secondary windings, wherein the magnetizing ACvoltage is such that responsive to the magnetizing AC voltage beingprovided to one of the number of sets of primary windings or one of thenumber of sets of secondary windings, one or more of the number of setsof primary windings will be magnetized in a manner wherein a voltage ofthe one or more of the number of primary windings is in phase with themain AC voltage provided from the AC source.
 9. A method of energizingan electrical system, the electrical system including a transformerstructured to be selectively coupled to an AC source that provides amain AC voltage having a main AC voltage level, the transformer having anumber of windings, the method comprising: generating a magnetizing ACvoltage from the main AC voltage having the main AC voltage level whenthe number of windings is not coupled to the AC source; providing themagnetizing AC voltage to the transformer when the number of windings isnot coupled to the AC source to magnetize one or more of the windings ina manner wherein a voltage of the one or more of the windings is inphase with the main AC voltage to eliminate or reduce in-rush currentinto the transformer; and coupling the windings to the AC source suchthat the main AC voltage is applied to the windings.
 10. The methodaccording to claim 9, further comprising terminating the providing themagnetizing AC voltage before or after coupling the number of windingsto the AC source.
 11. The method according to claim 9, wherein thegenerating the magnetizing AC voltage comprises providing the mainvoltage to an auxiliary transformer, and wherein the providing themagnetizing AC voltage comprises the auxiliary transformer to thetransformer when the number of windings is not coupled to the AC source.12. The method according to claim 11, wherein the transformer is awye-delta transformer and the auxiliary transformer is a delta-wyetransformer.
 13. The method according to claim 9, wherein thetransformer is an isolation transformer of a variable frequency drive.14. The method according to claim 9, wherein the number of windingscomprises a number of sets of first windings and a number of sets ofsecond windings.
 15. The method according to claim 14, wherein thenumber of sets of first windings is a number of sets of primary windingsand the number of sets of second windings is a number of sets ofsecondary windings.